1. Field of the Invention
The present invention relates to a belt-type continuously variable transmission that controls a gear change ratio by changing a winding radius of a belt wound around a pair of pulleys. Particularly, the present invention relates to a hydraulic control device of a belt-type continuously variable transmission that changes a gear change ratio by controlling hydraulic pressure of a hydraulic actuator attached to a pulley.
2. Description of Related Art
A following belt-type continuously variable transmission has been known as related art. The belt-type continuously variable transmission controls a gear change ratio by changing a winding radius of a belt that is wound around a primary pulley and a secondary pulley. The winding radius of the belt wound around the primary pulley and the secondary pulley is changed by controlling hydraulic pressure or an amount of oil of a hydraulic actuator that is attached to the primary pulley or the secondary pulley. In addition, in the belt-type continuously variable transmission that includes the hydraulic actuator attached to each of the pulleys and a control valve for controlling the hydraulic pressure or the amount of oil of the hydraulic actuator, the gear change ratio is controlled by supplying the oil to or discharging the oil from the hydraulic actuator. The hydraulic actuator pressurizes or depressurizes the oil by electrically controlling a solenoid valve that applies signal pressure to the control valve.
A belt-type continuously variable transmission disclosed in Japanese Patent Application Publication No. 11-247981 (JP 11-247981 A) is provided with a hydraulic actuator for each pulley to change a gear change ratio or a transmission torque capacity. The gear change ratio or the transmission torque capacity is changed by controlling hydraulic pressure of the each hydraulic actuator. More specifically, a spool-type hydraulic pressure control valve on which signal pressure output from a solenoid valve and feedback pressure of the hydraulic actuator are exerted is provided in an oil passage that communicates with the each hydraulic actuator. The hydraulic pressure of the each hydraulic actuator is controlled by controlling output pressure of the solenoid valve. In addition, in the belt-type continuously variable transmission disclosed in JP 11-247981 A, the output pressure of the solenoid valve that controls the hydraulic pressure of the primary pulley is lowered for a downshift, while the output pressure of the solenoid valve is increased for an upshift.
The hydraulic pressure control valve of JP 11-247981 A is a spool-type pressure regulator valve. The pressure regulator valve is opened or closed in accordance with the signal pressure and the feedback pressure. When the pressure regulator valve lowers the hydraulic pressure of the hydraulic actuator, the hydraulic pressure of the hydraulic actuator is lowered after the hydraulic pressure near the pressure regulator valve is lowered. Accordingly, there is a possibility that the feedback pressure exerted on a spool becomes lower than the hydraulic pressure of the hydraulic actuator. In addition, because the spool is moved on the basis of the feedback pressure and the signal pressure, there is a possibility that an opening area of the pressure regulator valve is reduced, thereby reducing a flow rate of oil to be discharged. In other words, when the hydraulic pressure of the hydraulic actuator is controlled by the hydraulic pressure control valve that regulates and outputs predetermined source pressure, there is a possibility that a change rate to increase or lower the hydraulic pressure of the hydraulic actuator is lowered. Therefore, a gear change speed for changing the gear change ratio may become slow.